Valve’s repair policy was exceptional with the steam deck so I have reasons to believe that the battery for the steam controller will be available for a very reasonable cost.
Just to piggy back on what you are say, one of the engineers in the LTT video mentioned they want to team up with iFixIt again, just like they did for the Steam Deck. And I saw the back shell off the controller in one of the videos. The batter looks dead simple to replace. It's wild to even imagine that a company in 2025 would be be consumer friendly.